1. Field of the Invention
The present invention relates to a frequency conversion method and a frequency conversion circuit.
Frequency conversion is one of most important processes in for example the field of radio communications. As one example of this, there is frequency conversion by an up-converter. In the present invention, a method and a circuit for performing such frequency conversion will be explained.
2. Description of the Related Art
Taking an up-converter as an example, when the frequency of the input signal is extremely low relative to the carrier frequency at the initial stage frequency conversion circuit and therefore the frequency difference between an upper sideband and a lower sideband becomes small, it becomes necessary to make use of a lower band erasing filter having a sharp cut-off characteristic and a high precision in order to eliminate the frequency components of the lower sideband and under without affecting the upper sideband thereof. However, such a high precision filter is usually expensive and thus there is a problem of a rise of the cost of the frequency conversion circuit.
Further, when the frequency of the input signal is extremely low relative to the carrier frequency, the desired low erasing filter cannot be realized, so the frequency components of the lower sideband and under cannot be eliminated by the conventional frequency conversion method without affecting the upper sideband. Accordingly, there is a problem that the desired frequency conversion cannot be carried out.